This invention relates to flame retardant materials, and in particular to a flame retardant material comprising metal hydroxide derivatives with chemically incorporated phosphorus-containing species.
Solid materials do not burn directly. They must first be decomposed by heat (pyrolysis) to release flammable gases. Visible flames appear when these flammable gases burn with the oxygen (O2) in the air. If solid materials do not break down into gases, then they will only smolder slowly and often self extinguish, particularly if they “char” and form a stable carbonaceous barrier which prevents access of the flame to the underlying material. However, materials such as wood do in fact burn vigorously, because once ignited the heat generated breaks down long-chain solid molecules into smaller molecules which transpire as gases. The gas flame itself is maintained by the action of high energy “radicals” (for example, H and OH in the gas phase) which act as intermediates in reaction sequences whose overall effect is reaction of the burning material with oxygen in the air to produce CO2 and water, thus generating heat energy. Thus, effective flame retardant materials often act to promote “charring” and/or to eliminate high energy radicals.
Metal hydroxides are among the most used flame retardants. This is due to a low cost compared to antimony-halogen systems and phosphorus-based flame retardants. Inorganic hydroxides are easily handled and relatively non-toxic. These classes of materials provide flame-retardant formulation that meet appropriate standards for many applications. Such formulations have low opacity, low toxicity, and minimal corrosivity.
Properly compounded, inorganic hydroxides offer a cost-effective way to achieve low-smoke flame-retardant formulations with low tendency for non-thermal damage. Aluminium trihydroxide (ATH) is the most commonly inorganic hydroxide used as flame retardant. It is suitable for elastomers and thermosetting resins, and thermoplastics processed below 200° C. Magnesium hydroxide (MH) is a more thermally stable inorganic flame retardant. It is stable above 300° C. and can be used in many elastomers and resins, including engineering plastics and other resins processed at higher temperatures.
Metal hydroxides contribute to several flame retardant actions. They first decompose endothermally and release water. The endothermic decomposition acts as a “heat sink” that cools the substrate and decreases the pyrolysis of the polymer (physical action in the condensed phase). The release of water dilutes the amount of fuel and avoids the critical fuel/oxygen-ratio (physical action in the gas phase). Both essentially fight against ignition. In some fire tests, the ignition has to be strongly delayed. Thus, metal hydroxides are suitable for these applications. Moreover, after the degradation, a ceramic-based-protective layer is created. This ceramic-based layer improves the insulating property (physical action in the condensed phase) and reveals a smoke suppressant effect (chemical action in the condensed phase). The ceramic-based protective layer ensures an efficient protection of the polymer during the combustion leading to a severe decrease of the heat release.
Layered double hydroxides (“LDH”), which may be referred to as hydrotalcite (“HT”) compounds, are used extensively as flame retardants. In particular, LDH as magnesium aluminum carbonate is used extensively as an additive to PVC. LDH works as a filler, undergoes endothermic decomposition in several steps, releases steam and carbon dioxide, and soaks up HCl from the decomposing polymer. The LDH material must also be treated to improve crystallinity and aid deaggregation and dispersion. LDH is capable of incorporating borate, trimetaphosphate, and anions containing phenyl and benzyl groups. These may act as potential char promoters and free radical terminators.
Phosphorus containing flame retardants usually act in the solid phase of burning materials. When heated, the phosphorus reacts to give a polymeric form of phosphoric acid (nominally HPO3). This acid causes the material to char, inhibiting the “pyrolysis” process (break down and release of flammable gases) which is necessary to feed flames.
Different phosphorus containing flame retardants can be either simply mixed into plastics (and then held in the material when the plastic sets) or be reactive, and chemically bind into the plastic molecules at polymerization. This will depend on the properties required of the plastic in terms of finished product performance, facility of processing (melting, extrusion, moulding) and flame retardancy (temperature of onset of the charring process). Phosphorus based flame retardants vary from elemental red phosphorus (P), which is oxidized to phosphoric acid with heat, to complex P-containing organic molecules offering specific performance properties. Certain products contain both phosphorus, halogen, and/or nitrogen, thus combining the different flame retarding mechanisms of these elements.
Phosphorus-containing additives refer to a wide range of compounds. These include halogenated phosphorus and non-halogenated phosphorus compounds. Non-halogenated phosphorus compounds include red phosphorus, water-soluble inorganic phosphates, ammonium phosphate and polyphosphate, organophosphates and phosphonates and phosphine oxide. Monoammonium phosphate and diammonium phosphate achieve good FR properties in a wide range of cellulosic-based materials such as textiles, woven or non-woven, paper and wood. Ammonium and melamine phosphates and polyphosphates, often mixed with pentaerythritol, behave similarly. The mechanism is a chemical one in condensed phase. The char formation results from esterification of hydroxyl groups with the phosphoric acid. However, the solubility of such additives leads to aging problems. Red phosphorus is a very efficient flame retardant for oxygenated polymers. Coated red phosphorus is commercially used as a flame retardant for nylon electrical parts. To inhibit its ignition in the air, red phosphorus is encapsulated in a high concentration of polymer. Therefore, commercial additives never contain more than 50 wt. %. Moreover, its toxicity and red color are the reasons why its use is not extensive.
Trialkyl phosphates and dialkyl phosphonates can also be used as flame retardants. Triethyl phosphate acts as a synergistic agent in halogenated polyesters but not as efficiently as antimony oxides. Dimethyl phosphonate is a very effective flame retardant due to its high phosphorus content. However, its high volatility limits its use in rigid PU and highly filled polyester. Aryl phosphates can be used as non-flammable plasticizers for PVC and cellulose acetate and as non-halogen flame retardants for modified PPO or PC/ABS blends. Triaryl phosphates are volatile during processing which leads to stress cracking. Use of less volatile diphosphates reduces processing problems.
What is needed, therefore, is a flame retardant material comprising the functionalities of both LDH and phosphorus-containing compounds, which is evenly dispersed in small particles into polymer materials and generates a crust at flame temperature.
Phenyl phosphonate has been incorporated into LDH, as has inorganic phosphate. The resulting materials have not been investigated for their flame retardant properties. Phosphorus-containing flame retardants have been mechanically mixed into polymers alongside LDH carbonate. This is not the optimum way of combining their properties, since it is necessary to achieve dispersal of two different materials of very different wetting and contact properties, and problems arising from the volatility or solubility of the phosphorus-containing component are not resolved.